An existing window frame edge connection fitting which is widely used is shown in FIG. 1.
FIG. 1 shows part of an edge portion at one side of a window frame. For convenience of understanding, a horizontal bar 2 and a vertical bar 3 which are somewhat complicated are simply represented in sectional shape. Protrusions 10 are inserted into the horizontal bar 2 and the vertical bar 3, respectively, in order to firmly couple an edge connection fitting with the horizontal bar 2 and the vertical bar 3, when the edge connection fitting is fitted with the horizontal bar 2 and the vertical bar 3. A rubber packing 12 is interposed between each of the horizontal bar 2 and the vertical bar 3 and the edge connection fitting 5, in order to enhance air-tightness, respectively. On the upper portion of the horizontal bar 2 are formed a raised step 6, a groove 13 and a packing insertion groove (not shown) in lengthy direction. Likewise, on the upper portion of the vertical bar 3 are formed a raised step 6, a groove 13 and a packing insertion groove (not shown) in lengthy direction. In assembly, a worker fits the cut end portions of the horizontal bar 2 and the vertical bar 3 with the edge connection fitting 5 and then makes holes such as piece holes or screw holes into the edge connection fitting 5 by using a drilling machine, to then tighten pieces or screws 8 into the piece holes or screw holes. In FIG. 1, a reference numeral 7 denotes a piece hole or screw hole.
FIG. 2 shows a coupled state of FIG. 1, in which the horizontal bar 2 and the vertical bar 3 are coupled with the edge connection fitting 5 according to the conventional art. A window frame is classified into a convex type and a concave type according to the shape of the window frame with which a window sash is fitted. FIG. 2 shows a convex type window frame. A window sash is slid on the raised step 6. A concave type window frame is fabricated in the same shape and fabrication method as that of the convex type window frame, although it is not shown in the drawings. FIG. 2 shows positions of the raised step 6, the groove 13, and the screw hole 7.
FIG. 3 is a perspective view showing a coupling relationship of a conventional window sash edge connection fitting 5a. A glass groove 17a is formed in the inner circumference of the window sash edge connection fitting 5a, and a packing insertion groove 11a is formed in the lower portion of the glass groove 17a. The window sash edge connection fitting 5a is coupled in the same pattern as that of the window frame edge connection fitting 5. That is, protrusion 10a of the window sash edge connection fitting 5a are fitted into a horizontal bar 2a and a vertical bar 3a, respectively, and tightened by screws 8a to then be assembled with the horizontal bar 2a and the vertical bar 3a. 
In FIGS. 2 and 3, the window frame edge connection fitting 5 and the window sash edge connection fitting 5a provide a coupling function which fits horizontal bars 2 and 2a and vertical bars 3 and 3a, respectively. Since the edge connection fittings 5 and 5a where the horizontal bars 2 and 2a and vertical bars 3 and 3a are crossed are coupled perpendicularly, the coupled modular window frame is weak with respect to a diagonally lateral compression force. Thus, in the case of windows at the windy seaside areas, high buildings, or seismal places, stronger connection fittings are required.
Also, the window frame edge connection fitting 5 and the window sash edge connection fitting 5a have no proper structures of packing insertion grooves 11a of FIGS. 2 and 3 into which packing members such as mohair called wind-proof hair (not shown) and gaskets made of soft rubber can be inserted.
That is, the horizontal bars 2 and 2a and vertical bars 3 and 3a are crossed are coupled perpendicularly, at the edge connection fittings 5 and 5a. Accordingly, packing members cannot be inserted into packing insertion grooves. Thus, the packing members are cut and used according to the lengths of the horizontal bars 2 and 2a, respectively. In the case of the vertical bars 3 and 3a, the packing members are also cut and used according to the lengths thereof. The packing members such as mohair called wind-proof hair (not shown) and gaskets made of soft rubber which are inserted into the horizontal bars 2 and 2a and vertical bars 3 and 3a are cut and used in proper lengths according to a skillfulness of a worker who performs a window assembly, and are fitted into the packing insertion grooves 11a. No matter how a worker is skill, he or she cuts packing members such as mohair called wind-proof hair (not shown) and gaskets made of soft rubber by eye measure in order to heighten a working speed, when the packing members are fitted into the packing insertion grooves 11a formed in the horizontal bars 2 and 2a and vertical bars 3 and 3a. As a result, the packing members may be cut shortly in comparison with the lengths of the horizontal bars 2 and 2a and vertical bars 3 and 3a, and thus it is difficult to insert the packing members such as mohair called wind-proof hair (not shown) and gaskets made of soft rubber which have been cut in proper lengths into the packing insertion grooves 11a formed in the horizontal bars 2 and 2a and vertical bars 3 and 3a. The packing members having been inserted into the packing insertion grooves formed in the horizontal bars 2 and 2a and vertical bars 3 and 3a reduce heat insulation and air-tightness, to thereby lower an energy efficiency and generate noise. Also, it is not so easy to clean dust collected in the perpendicular edges.
FIG. 13 is a perspective view of a typical casement which is one of casements. FIG. 14 is a perspective view of a tilt which is one of casements. FIG. 15 is a lateral cross-sectional view of a project which is one of casements. An edge connection fitting for a modular casement is not much developed in comparison with that of a slider. However, in the case of a slider which is opened in the form of protruding inwards or outwards in view of structure of a window, edge portions of a window sash 1a have a sharp point, respectively. This, if an article or person collides with the pointed edges due to carelessness, the article may be broken or the head or face of the person may be damaged severely. The protruded edge portions of synthetic resin window, wooden window, and hybrid window are always dangerous. In particular, since the edge portions of an aluminum window are sharp like a knife blade, an article or the head or face of person colliding with the pointed edges due to carelessness may be broken or damaged severely. Such an accident frequently takes place.